A light emitting device is a device in which electric energy is converted into light energy. For example, a light emitting diode (LED) can realize various colors by controlling a composition ratio of a compound semiconductor. For example, nitride semiconductors have received great interest in optical devices and high power electronic devices due to their high thermal stability and wide bandgap energy. In particular, a blue light emitting device, a green light emitting device, and an ultraviolet (UV) light emitting device and the like using a nitride semiconductor are commercially available and widely used.
LED that emits white light is a method of using a secondary light source which emits light from a phosphor by applying a phosphor, and a method of obtaining a white light by applying a YAG:Ce phosphor that emits yellow to a blue LED is generally used. However, this method is disadvantageous in that the efficiency due to quantum deficits and re-emission efficiency generated by using the secondary light is accompanied and color rendering is not easy.
Therefore, a conventional white LED backlight is a combination of a blue LED chip and a yellow phosphor, and is limited in the extent that it can be used for a screen of a mobile phone or a notebook PC because green and red components are lacking to represent unnatural colors. Nevertheless, it is widely commercialized because of its advantages of being easy to drive and remarkably inexpensive.
The use of a white LED device made of a blue chip and a yellow phosphor is for a backlight used in a mobile phone, a personal digital assistant or the like. However, due to the use of yellow phosphors (e.g., the peak emission appears between 550 nm and 610 nm), the spectrum of the LED contains excessive emission in the yellow region of this spectrum, which strongly reduces the color gamut of the backlight.
The color gamut is an area spanned between chromaticity points of the red, green, and blue pixels of the display in chromaticity, e.g., CIE 1931 x, y chromaticity. The historical “golden standard” of the display is the NTSC gamut, which is limited to a set of three color coordinates. In general, a full range exceeding 70% of NTSC is considered acceptable for many backlighting applications, and a full range exceeding 90% of NTSC is considered acceptable for most of these optional uses.
When the entire range of the LED backlight is improved by using the yellow phosphor, the yellow light is removed, thereby reducing the brightness of the LED backlight. Therefore, it would be beneficial to develop a white LED that can increase the brightness without using a yellow phosphor in the package.
Although the CRI is commonly referred to as the General Color Rendering Index and is abbreviated as Ra, although it is internationally specified that 14 of standard color samples are internationally available and can calculate a wider CRI R1-R14 as their mean value, is generally defined as the average value of color samples R1-R8. Specifically, a R9 value for measuring the color rendering property against a strong red color is very important for various applications, particularly in lighting and medical fields requiring a high color rendering index.